The Cleveland Museum of Art agreed Wednesday to return 13 antiquities and a late Gothic processional cross to Italy after authorities there proved the works were looted, stolen or handled by traffickers.

The museum and the Ministry for Cultural Assets and Activities announced the agreement at a news conference in Rome 3 p.m. Wednesday (9 a.m. in Cleveland).

The accord, the first of its kind between the Cleveland museum and a foreign country, concludes what officials on both sides called a friendly and collaborative 18-month negotiation.

“I’m very pleased,” Timothy Rub, director of the museum, said Wednesday, speaking by phone from Rome. “This has been a very open and thoughtful discussion.”

Maurizio Fiorilli, the Italian state lawyer who represented the ministry in the negotiations, praised Rub and the Cleveland museum.

“The director is an exquisite person,” Fiorilli said by phone from Rome. “This was a negotiation among gentlemen. They always collaborated and exhibited great openness; therefore, I am content.”

Rub signed a document formalizing the agreement with Giuseppe Proietti, the secretary general of the Italian cultural ministry. Sandro Bondi, the head of the agency, also attended the signing, along with Fiorilli.

The accord is the latest of a series in which Italy has persuaded American museums to return antiquities unearthed by tomb robbers, or “tombaroli,” and sold ultimately through middlemen to unwitting buyers.

The J. Paul Getty Museum in Los Angeles, the Museum of Fine Arts Boston and the Metropolitan Museum of Art in New York have all agreed to return scores of artworks.

Malcolm Bell III, a widely respected art historian at the University of Virginia in Charlottesville who has supervised archaeological excavations at Morgantina in Sicily, hailed the Cleveland agreement.

He said that accords like it, combined with tougher law enforcement in Italy, are having an impact.

“The uncontrolled looting has diminished partly because the market is much weakened,” he said.

“The rate of looting has decreased in recent years.”

The agreement with Cleveland underscores that the museum innocently acquired objects that “clearly were associated with bad actors” at some point in their past, Rub said. He said the agreement acknowledges that neither the museum nor its curators past and present are considered at fault.

Perhaps the most significant object to be returned is a fourth-century B.C. Apulian red-figured volute krater by the Dorias Painter, which stands roughly 4 feet high. (A krater is a large vessel with handles used to hold wine.)

Other works include Etruscan silver bracelets; a Neolithic Sardinian bronze representing a warrior; an Attic rhyton, or drinking vessel, in the shape of a mule; and a large Corinthian column krater.

The Cleveland deal includes Italy’s promise to lend the museum 13 antiquities similar to those being returned, for renewable 25-year periods. Italy will collaborate on an exhibition of artworks from state museums and other initiatives.

Rub declined to describe the value of the 13 works, which were bought or donated between 1975 and 1996.

But he said the agreement with Italy “will allow us to show a very broad range of Greek and Italian antiquities and to do research and exhibitions in the field, working with Italy.”

The 14th object covered by the agreement is a 14th-century processional cross from Trequanda, a small town outside Siena. It was stolen from a local church after World War II and will be returned by the Cleveland museum as a gift to the church, Fiorilli said. The museum bought the work in 1977.

Italy and the museum also agreed to form a joint scientific commission to perform further research on the large ancient bronze statue of Apollo acquired by the museum in 2004, plus an additional small ancient bronze chariot ornament in the form of a winged victory.

The Apollo has been a subject of controversy because it lacks a complete provenance, or ownership history. Greece charged in early 2007 that the work had been fished out of Italian waters, but offered no proof and has not pressed a claim.

The museum has said the work belonged to a collection in the former East Germany before World War II. The museum also says that scientific evidence shows the work was excavated at least 100 years ago, and was never in contact with seawater.

If Italian scientists feel that the museum’s explanations don’t hold up after new tests, the Apollo could become the subject of a fresh round of discussions, Fiorilli said.

Italy launched its negotiations with the Cleveland museum in 2007 by presenting a list of 42 objects about which government authorities sought information. They included the Apollo and a widely admired South Italian Medea vase.

The list ultimately was whittled to 14, including many from South Italy, formerly a hotbed of looting.

Fiorilli said that evidence from a 1995 police raid on a Swiss warehouse, including photographs and letters, pointed to some of the Cleveland works. Subsequent investigations strengthened Italy’s case.

The evidence linked the Cleveland artworks to convicted antiquities smuggler Giacomo de Medici and others in his circle, Fiorilli said.

Those included the American art dealer Robert Hecht, the English dealer Robin Symes and Fritz Burki, a Swiss art restorer close to Medici.

Rub also said the museum purchased all the artworks in question after the 1970 UNESCO convention aimed at halting illegal trade in antiquities.

The Cleveland museum hasn’t faced claims other than this one from Italy, Rub said.

“Our experience has been, and I say this in all sincerity, very positive,” Rub said.

“The representatives of the Italian government we’ve worked with have been dedicated to their work and to righting what they perceive as wrongs, as well they should.

“But they’ve also conducted these conversations reasonably and in a very thoughtful manner. We’ve looked at things together and come to conclusions that both sides believe are fair and equitable.”